Slight miss/exhaust backfire when cold since Headers installed

[66 Sat]

So I collected my car this morning after having custom headers installed. Engine is a 354 cu poly stroker, 10.8:1, Ross pistons, 600 Holley vac sec, manual choke wired open (have never needed it in this hot climate).

Car started fine and idled ok but after a few minutes driving I noticed a slight surging or missfire. I pulled over and checked all the plug wires were on and were not crossed over - all good. I continued driving home and car started backfiring occasionally through the exhaust at a light throttle (2,250 rpm). Once the car was fully warm it went away and was fine for the remaining 45 minutes on the freeway at 3,000rpm. I did pull off the freeway to be able to get back on from a standing start - wow, the headers have really opened up this engine - again no issues. It was also fine cruising the last few miles on a light throttle from the freeway to my house too.

Questions:
1. Is the surging/backfire because of a lean condition since the headers were fitted, made worse on a cold engine? The old manifolds were very restrictive and the engine was being strangled. If so how do I fix it?
2. Will I do any damage driving it around like this until it's fixed, bearing in mind it only seems to do it from cold (although I can't confirm this yet as I've only driven it once)?

I'm not an expert in tuning at all.

Thanks

 

[Kern Dog]

The change to headers often does result in a lean condition. An air/fuel gauge would tell you for sure.
The carburetor jets need to be changed to a higher number. Some suggest to go up two sizes and test drive it.

 

[66 Sat]

In the short term can I just richen the mixture by adjusting the mixture screws? I'm a total amateur when it comes to carburetors.

 

[Kern Dog]

The mixture screws only make a difference at closed throttle conditions or with very little throttle opening. The main jets affect the light acceleration and cruise conditions. The 600 probably does not have secondary jets so you are limited on the adjustability with that carburetor in terms of wide open throttle conditions.
I run a 600 on a basically stock 360 in my 67 Dart. It runs pretty strong for what it is.

 

[WileERobby]

The mixture screws are IDLE mixture adjustments. After throttle opens, the carb transitions to the main metering circuit - that's where you'd change the jets to a richer mixture.

 

[66 Sat]

Well I changed the jets from the .066 that were in there to .068.
The 68's looked the same size as the 66's and I thought maybe the guy that has tuned my car in the past had drilled them out, so back to the store and bought some .070. Then I noticed the rear of the jets seem to be deeper on the 68's ( and the 70's) and it's not just the hole size being different. Is this correct? I put the 68's in and will try it in the morning. If no change I'll go to the 70's.
Anyway, I couldn't believe how easy it was to change the jets, I grew up with fuel injection so never got to play with carbs when I was younger.

 

[Kern Dog]

Two statements about Holley carburetors that are equally true:

1) Holley carburetors are built with a variety of tuning aids including jets, accelerator pumps, air bleeds, power valves and secondary springs that allow the owner to make their car run great.

2) Holley carburetors are built with a variety of tuning aids including jets, accelerator pumps, air bleeds, power valves and secondary springs that can cause the owner no end of frustration.

 

[66 Sat]

I've never had any dramas with Holley, on my old Barracuda, my 351 Cleveland Aussie Ford Falcon or my Satellite. Some people seem to hate them, but often they've bought some used Holley from a swap meet and then complain it's a piece of junk. I've always had new ones and had no issues. Are they the largest manufacturer of carburetors in the world? Maybe. You would think they would know what they are doing.

 

[moparedtn]

Holley or Edelbrock/Carter - eternal debate. Both have advantages/disadvantages....
The symptoms the OP describes can and do happen to either brand and the solutions will be
similar in a given situation no matter which is on the car now.

For street driving, follow manufacturers' recommendations for your carb and continue until the
driveability is where you want it.

 

[dvw]

It is very likely that on low rpm cruise you ARE running on the idle circuit. Main jets and power valves do NOT affect the idle mixture. Yes it could be lean due to jetting as well. Yuo can't see the difference in a 66 to 68 jet by eye. Another thing to consider. The plugs were most likely removed during header install. It's possible that one may have a hairline crack.
Doug

 

[snake]

take a look at the header tube too close to your power stealing hose going too cook the PS fluid.

 

[PurpleBeeper]

My 2-cents? You just put on headers. Your plug wires "look" like they're too close to the headers on the driver's side. Headers burn up plug wires quickly. Also, for new wires, often the plug end pops off the spark plug (happened to me many times with new wires). So.....I'd suggest you take a hard look at your plug wires and maybe get longer ones and/or add heat shielding (if burnt) before you start changing jets. Just my 2-cents

 

[66 Sat]

The headers are ceramic coated (polished) which should help with the temperatures/burning leads, but I'll have a look at it this morning and maybe wrap them in foil until I can get some proper heat shields.

The power steering fluid didn't boil on the 60 mile trip home but I'll look at insulating that too.

 

[66 Sat]

The plug wires are actually at least an inch away from the headers which is good news, the photos are deceiving. I will do something about the PS hose though, even rotating the hose clamp will help to bring the nut away.

 

[66 Sat]

Well Kern Dog was right. With the bigger jets the car runs like a champ, no miss/backfire from cold. Thanks for all the help guys.